Depends on how good you want it to look - some of advice above makes it pretty easy to land it correctly.


I try to *never* use digital keystone correction - don't like it in theory or practice. It's like throwing "X" number of pixels in the garbage can right of the box and then going downhill from there. Optical Lens Shift seems OK.

I've always heard to try to keep the lens zoom from either extreme, but I've definitely done it.

This message was edited by Barry Shaw on 04/26/05 13:52 ET.

That will give you a center point, but there is no guarantee that this point will be at the distance from the screen that you want.

What you want to do is do this twice, with, say, 12 feet, then again with 13 feet. This will give you two points, each of which is centered on the screen. Draw a line from one point to the other and you will have the center line for the lens as well as a line that should be parallel to the side of the projector.

Keep in mind when you set this up that
a)the way you hold the tape measures must be symmetrical on the two sides to get validly equal measurements;
b)you can do this with two strings with knots; you really don't need inches to mark these center points.

In fact, I do this with ONE tape measure because it is the same length no matter which side of the screen I am on. I put a piece of blue tape on the ceiling where I think the middle is, as parallel to the screen as I can see, then make marks on the tape where some random inch mark comes from the left, and the same inch mark from the right. The middle between those two is the center.

This takes two people. The two-tape-measure approach takes three people.

I hate digital keystone too.... but since JVC D-ILA lacks a lens shift, it has been a necessary evel for me on a couple jobs.
It was a toss up between reduced aesthetic appeal (a PJ dangling from a pole mount) or a slight compromise in performance.

I think I made the right choice at the time.

Walls and ceilings are only human, so I NEVER measure from the walls. To do so assumes they are dead ninety degrees, that the ceiling and floor are dead level, and that the screen has been mounted dead parallel to the wall.

Since zoom and keystone modify the default output of the projector, I think they should be used to customize the mounting position (i.e. ceiling much higher than screen, can use keystone to keep from having the projector sit 12" down from the ceiling and look like hell). Or correct for dumb mistakes.

I learned this with Vidikron 3-gun projectors, where ALL installations require lots of keystone, pinushion, etc adjustments. With those, if the projector is too close, the corners of the image will be off the tube, so will disappear. If they are further than the one proper distance, the image must be made smaller on the tube, so the image just gets darker and darker as you back up the projector! Then, if you have installed it where you have to tweak the hell out of the image to square it up on the screen, it can happen that normal service adjustments will take you to the point where you are out of memory, and you will have to go back and spend hours converging the projector, starting with green geometry!

By the way, vwpower44, don't worry about being "to anal." If you were truly "to anal," you would write "too anal" and there would be zero "teh"s in your posts....

Sorry about the typos. The keyboard is my arch-nemesis.

Mike

my opinion:

zoom: I like the middle, gives some playing room
keystone: avoid as much as possible.
lens shift: is OK, no problem using it. On the other hand I did see a projector once (can't remember what it was) that had a digital shift, and that is as bad as keystone.

We use a cheap laser level screwed into a framers square. Mark the center point of the screen , hold the square flat to the screen and mark the ceiling on some blue painters tape at desired throw/distance.

That's the simplest GREAT improvement I have ever heard of.

Be sure the framers square is level from side to side. If one side of the level droops, the laser line will not go straight out from the screen unless the part pointing toward the projector position is dead level. And chances are that if something about the way you do it makes it impossible to hold one part leve, you won't be able to keep the other part level either.

Ernie, thats correct you definately have to make sure the edge of the square that is against the screen is level or at least snugged up against a the frame of the screen which should be level.